# std::atomic_wait, std::atomic_wait_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
template< class T > void atomic_wait( const std::atomic<T>* object, typename std::atomic<T>::value_type old ); | (1) | (since C++20)
template< class T > void atomic_wait( const volatile std::atomic<T>* object, typename std::atomic<T>::value_type old ); | (2) | (since C++20)
template< class T > void atomic_wait_explicit( const std::atomic<T>* object, typename std::atomic<T>::value_type old, std::memory_order order ); | (3) | (since C++20)
template< class T > void atomic_wait_explicit( const volatile std::atomic<T>* object, typename std::atomic<T>::value_type old, std::memory_order order ); | (4) | (since C++20)

Performs atomic waiting operations. Behaves as if it repeatedly performs the following steps:

- Compare the value representation of object->load() (for overloads (1,2)) or object->load(order) (for overloads (3,4)) with that of old.
  - If those are bitwise equal, then blocks until *object is notified by std::atomic::notify_one() or std::atomic::notify_all(), or the thread is unblocked spuriously.
  - Otherwise, returns.

These functions are guaranteed to return only if value has changed, even if the underlying implementation unblocks spuriously.

1,2) Equivalent to object->wait(old).
3,4) Equivalent to object->wait(old, order).
If order is one of std::memory_order::release and std::memory_order::acq_rel, the behavior is undefined.

### Parameters

object | - | pointer to the atomic object to check and wait on
old | - | the value to check the atomic object no longer contains
order | - | the memory synchronization ordering

### Return value

(none)

### Notes

This form of change-detection is often more efficient than simple polling or pure spinlocks.

Due to the ABA problem, transient changes from old to another value and back to old might be missed, and not unblock.

The comparison is bitwise (similar to std::memcmp); no comparison operator is used. Padding bits that never participate in an object's value representation are ignored.

### Example

| This section is incomplete Reason: no example

### See also

wait (C++20) | blocks the thread until notified and the atomic value changes (public member function of `std::atomic<T>`)
notify_one (C++20) | notifies at least one thread waiting on the atomic object (public member function of `std::atomic<T>`)
notify_all (C++20) | notifies all threads blocked waiting on the atomic object (public member function of `std::atomic<T>`)
atomic_notify_one (C++20) | notifies a thread blocked in atomic_wait (function template)
atomic_notify_all (C++20) | notifies all threads blocked in atomic_wait (function template)

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